A dielectric elastomer (DE) can be used to convert electrical energy into mechanical energy, or vice versa. DEs can be used as actuators, generators, and/or sensors.
Referring to FIGS. 1(a) and 1(b), a DE (generally referenced 10) typically comprises a volumetrically incompressible dielectric elastomer membrane 11 provided between compliant electrodes 12.
Dielectric Elastomer Actuators (DEA) deform when a voltage difference is applied across the electrodes 12, as disclosed in “Electrostriction of polymer dielectrics with compliant electrodes as a means of actuation” (Pelrine R. E., Kornbluh R. D., & Joseph J. P., Sensors and Actuators, A: Physical 64(1), pp 77-85 (1998)), for example. Electrical charge accumulates on the electrodes 12 in the manner of a capacitor and an electrostatic pressure is generated that results in a through-thickness compression and in-plane expansion of the membrane, from the rest or contracted state as shown by example in FIG. 1(a), to the actuated or expanded state illustrated in FIG. 1(b).
If the planar dimensions of the DEA are much greater than its thickness, the magnitude of the pressure is defined by the equation below, where P is the pressure, ∈r is the relative permittivity of the dielectric material, ∈0 is the permittivity of free space (8.854×10−12 F/m), V is the voltage, and d is the dielectric membrane thickness.
  P  =            ɛ      r        ⁢                            ɛ          0                ⁡                  (                      V            d                    )                    2      
A dielectric elastomer generator (DEG), on the other hand, converts mechanical energy into electrical energy. In effect, the DEG is a variable capacitor, and its capacitance changes with mechanical strain (i.e. deformation of the membrane).
The DEG generates electrical energy by increasing the electric potential energy stored in it. The steps to achieve this are illustrated diagrammatically in FIG. 2. Starting from the top of FIG. 2, mechanical energy 20 is initially applied to the DEG 21 by stretching it. This results in a planar expansion of electrodes 22 and an orthogonal compression of the membrane 23, leading to an increased capacitance. Electrical energy 24 is then input to the DEG by charging (or priming) from an electric power source (not shown) so that opposing electrodes 22 become oppositely charged. Relaxing the DEG will convert the mechanical energy into electrical energy by forcing apart the opposite charges (+ and −) on opposing electrodes 22, and forcing the like charges on each electrode 22 closer together due to the planar contraction thereof. The electrical energy 24 is extracted and the cycle repeats.
Dielectric elastomers may alternatively, or additionally, be used as a sensor. The electrical properties of dielectric elastomer sensors such as capacitance, electrode resistance, and the equivalent parallel resistance of the dielectric membrane change in response to external stimuli such as mechanical deformation or changes in the DE's operating environment, for example. Changes in the electrical parameters of the DE can therefore be used to acquire feedback information.
For many practical applications of DEAs, DEGs, and DE sensors, it is generally necessary or at least preferable to obtain feedback regarding the instantaneous state of the DEA for sensing and control purposes. For DEA and DEG, coupling them directly to discrete rigid strain, displacement, velocity or acceleration sensors inevitably increases the cost and mass of the device, and while such a strategy is effective for traditional, rigid devices that are part of kinematically constrained structures, it unnecessarily inhibits the motion of DEA/DEG. Similarly, attaching a sensor to a rigid body to which the DE is connected may provide useful feedback regarding the mechanical output, but provides limited information regarding the electromechanical state of the DE itself. The key therefore is to use the DE itself as a sensor, and combine sensing with actuation and/or generation, henceforth referred to as a “self-sensing” capability.
It is possible to relate a change in capacitance of the DE to changes in the physical geometry of the DE. Self-sensing efforts in the prior art have focused on combining sensing with actuation. “An adaptive control method for dielectric elastomer devices” (Todd A. Gisby, Emilio P. Callus, Shane Xie, and lain A. Anderson, Proc. SPIE, 2008) discloses the use of self-sensing based upon the capacitance between electrodes to determine the state of a DEA, thereby providing some feedback. Similar methods are disclosed by “Control system design for a dielectric elastomer actuator: The sensory subsystem” (Toth, L. A. and A. A. Goldenberg, Proceedings of SPIE, 2002), “Capacitive extensometry for transient strain analysis of dielectric elastomer actuators” (Keplinger, C., Kaltenbrunner, M., Arnold, N., and Bauer, S., Applied Physics Letters, 2008) and “A self-sensing dielectric elastomer actuator” (Jung, K., K. J. Kim, and H. R. Choi, Sensors and Actuators A: Physical, 2008), for example.
International Patent Publication No. WO 2010/095960 entitled “System and Method for Dynamic Self-Sensing of Dielectric Elastomer Actuators” discloses an improved method and system for self-sensing by deriving an estimate of the capacitance of a DEA from the electrical potential difference across the capacitance of the DEA, the rate of change of that potential difference, and the current through the DEA. From the capacitance, estimates of the charge and physical state of the DEA can be derived. It is also possible to derive an estimate of the leakage current through the dielectric membrane.
However, this and other self-sensing methods of the prior art typically have one or more disadvantages. These may include the estimates being accurate only under certain circumstances, such as when the DE is stationary (i.e. not subject to any perturbations caused by external forces), the leakage current is negligible, or at low actuation speeds. Alternatively, or additionally, the methods and/or apparatus of the prior art obtain estimates based upon other assumptions which may not always hold true, are not suitable for practical implementation in a system designed for portable use, are sensitive to noise, and/or require unnecessarily complex, numerous, or processor-intensive calculations to be performed.